1. Technical Field of the Invention
The present invention relates generally to a gas concentration sensor for measuring the concentration of gases which may be employed in an air-fuel ratio control system for automotive vehicles, and more particularly to a limiting current type gas concentration measuring apparatus equipped with a limiting current type gas sensor which is designed to compensate for an error in determining the concentration of a gas.
2. Background Art
Limiting current type gas concentration sensors are known which are used for measuring NOx contained in exhaust gasses of automotive engine. There is used one of such gas concentration sensors which includes a pump cell and a sensor cell. The pump cell works to pump oxygen (O2) contained in gasses admitted into a gas chamber out of the sensor or to pump oxygen (O2) of outside gasses into the gas chamber. The sensor cell works to measure the concentration of NOx contained in the gasses after passing through the pump cell. The pump cell and the sensor cell are designed to produce current signals indicative of the concentration of oxygen and NOx upon application of voltage thereto.
Another type of gas concentration sensor is known which includes a monitor cell in addition to the pump cell and the sensor cell. The monitor cell works to produce an electromotive force as a function of the concentration of oxygen within the gas chamber. A control system is also proposed which controls the voltage to be applied to the pump cell of such a three-cell gas concentration sensor under PID feedback (e.g., Academic Lecture Preliminary Report, Automotive Technical Meeting Corporation and SAE 970858). Specifically, this system is designed to determine the voltage to be applied to the pump cell based on a difference between an actual electromotive force produced by the monitor cell and a target one predetermined for keeping the concentration of oxygen at a lower level within the gas chamber.
The measurement of the concentration of exhaust gasses in the three-cell gas concentration sensor is achieved by introducing the exhaust gasses from the pump cell toward the monitor cell. Thus, when the concentration of exhaust gasses varies, a difference in concentration of the exhaust gasses between the pump cell and the monitor cell is resulted from a lag caused by the time required for the exhaust gasses to flow from the pump cell to the monitor cell. The time will, therefore, be consumed until the concentration of the exhaust gasses at the monitor cell agrees with that at the pump cell. This problem will be objectionable in a gas concentration sensor in which an orifice is provided between the pump cell and the monitor cell. Accordingly, when the voltage to be applied to the pump cell is feedback controlled as a function of the electromotive force produced by the monitor cell, a shift in feedback control phase may result in oscillation.
For instance, when it is required to change the exhaust gasses to a lean side, so that a large quantity of oxygen flows into the gas chamber, it is difficult for the monitor cell to detect such changing, thereby resulting in insufficient quantity of oxygen pumped by the pump cell under the feedback control using an output of the monitor cell. A large quantity of oxygen, thus, remains undesirably within the gas chamber. Subsequently, when the lean condition of the exhaust gasses is detected based on the output of the monitor cell, it will cause an excess voltage to be applied to the pump cell to pump the residual oxygen out of the gas chamber thereinto, after which the monitor cell continues to provide an output indicative of the lean condition for a while. After it is found that too much oxygen has been pumped out of the gas chamber, the voltage applied to the pump cell is change rapidly to a lower level.
The above phenomenon is repeated, thereby leading to oscillation of a control system applying the voltage to the pump cell, so that a residual quantity of oxygen within the gas chamber changes greatly in a cycle. This may cause the quantity of oxygen contained in the exhaust gasses flowing to the sensor cell to increase or the pump cell to decompose NOx undesirably. When the former is taken place, the sensor cell decomposes the increased quantity of oxygen to increase an offset current contained in an output thereof. When the latter is taken place, it results in insufficient quantity of NOx contained in the exhaust gasses flowing to the sensor cell, thus producing an error in determining the concentration of NOx.
Additionally, when the response rate of each cell is changed with a change in temperature of the exhaust gasses or deterioration of the cell, a residual quantity of oxygen within the gas chamber also changes, thus resulting in a decrease in accuracy of determining the concentration of NOx. For instance, in a case where enriched exhaust gases are admitted into the gas chamber, and a rich gas component (e.g., HC) sticks to an electrode of the monitor cell, the monitor cell continues to produce an output indicative of the rich condition in error even after the exhaust gasses is changed to the lean side. This causes the voltage applied to the pump cell to be controlled so that oxygen (O2) of outside gasses may be pumped into the gas chamber. Afterwards, when the rich gas component sticking to the electrode of the monitor cell reacts with the oxygen and peels, the monitor cell produces an output indicative of changing to the lean side, so that the voltage to be applied to the pump cell is so controlled as to pump the oxygen out of the gas chamber. At this time, the concentration of oxygen within the gas chamber is increased extremely, thus resulting in application of an excess voltage to the pump cell. Similarly, in a case where the exhaust gases are switched from lean to rich, and a lean gas component (e.g., O2) sticks to the electrode of the monitor cell, the monitor cell continues to produce an output indicative of the lean condition in error even after the exhaust gasses is changed to the rich side. This causes the voltage applied to the pump cell to be controlled so that oxygen (O2) may be pumped out of the gas chamber. Specifically, an excess voltage is applied to the pump cell so as to decompose NOx as well as O2.
The above event is repeated, thereby leading to oscillation of the control system applying the voltage to the pump cell, so that a residual quantity of oxygen (O2) within the gas chamber changes greatly in a cycle. This results in decreased accuracy of determining the concentration of NOx.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a gas concentration measuring apparatus designed to eliminate an error in determining the concentration of a specified gas component of measurement gases.
According to one aspect of the invention, there is provided a gas concentration measuring apparatus which may be employed with an automotive control system designed to control the quantity of fuel injected into an internal combustion gasoline engine as a function of an output of the gas concentration measuring apparatus under feedback (F/B) control to bring the air-fuel (A/F) ratio into agreement with a target value. The gas concentration measuring apparatus comprises: (a) a gas concentration sensor including a gas chamber, a first cell responsive to application of a voltage to pump thereinto oxygen molecules contained in gasses admitted into the gas chamber and discharge the pumped oxygen molecules to produce an electric current indicative of a concentration of the oxygen molecules, a second cell working to determine a concentration of a specified oxygen containing gas component contained in the gasses having passed through the first cell, and a monitor cell working to monitor a concentration of residual oxygen molecules within the gas chamber and provide an output indicative thereof; (b) an applying voltage determining circuit looking up a predetermined voltage-to-current relation to determine a target voltage to be applied to the first cell as a function of the electric current produced by the first cell so as to preclude the first cell from decomposing the specified oxygen containing gas component; and (c) an applying voltage controlling circuit working to apply the target voltage determined by the applying voltage determining circuit to the first cell.
Unlike a conventional system in which an electromotive force produced by a monitor cell is brought into agreement with a target one under feedback control using PID techniques, the use of the voltage-to-current relation to determine the target voltage to be applied to the first cell in the gas concentration measuring apparatus of the invention eliminates the problem that the applying voltage controlling circuit oscillates due to a response delay of the monitor cell, thereby resulting in a great cyclic change in residual quantity of oxygen within the gas chamber.
In a conventional system in which the voltage to be applied to the first cell is determined using an output of the monitor cell under feedback control, the voltage applied to the first cell and outputs of the first cell, the second cell, and the monitor cell when the concentration of gasses changes to a lean side so that the concentration of oxygen increases are varied as shown in FIG. 22(b). Specifically, an undesirable time is consumed in detecting a change in concentration of gasses using the output of the monitor cell. The avoidance of oscillation arising from a difference in response speed between the first cell and the monitor cell requires decreasing a feedback gain, so that the voltage applied to the first cell changes slowly, which results in a lack of discharge of oxygen from the gas chamber. An excess current, thus, flows through the second cell. It is, in practice, difficult to determine the concentration of gasses until the outputs of the second cell and the monitor cell are in steady state.
In the gas concentration measuring apparatus of the invention, when the current produced by the first cell changes with a change in concentration of the gasses, the voltage applied to the first cell is, as can be seen from FIG. 22(a), adjusted to a target one immediately, thus preventing the concentration of oxygen from being increased undesirably within the gas chamber, so that the output of the monitor cell remains unchanged. The second cell, thus, produces the current indicative of the concentration of the specified oxygen containing gas quickly.
In the preferred mode of the invention, the gas chamber includes a first chamber to which the first cell is exposed, a second chamber to which the monitor cell is exposed, and a diffusion path communicating between the first and second chambers.
The predetermined voltage-to-current relation is listed in a map. The applying voltage determining circuit determines the target voltage to be applied to the first cell by look-up using the map.
An applying voltage correcting circuit may further be provided which works to correct the target voltage to be applied to the first cell as a function of a given residual oxygen variation factor of a variation in residual quantity of oxygen within the gas chamber after the first cell pumps the oxygen molecules. The residual oxygen variation factor is, for example, a change in concentration of the gasses, a change in activity of the first cell, or an inherent error of the sensor.
The applying voltage correcting circuit may correct the target voltage based on the output of the monitor cell.
The first cell is formed in a solid electrolyte element. A resistance measuring circuit may also be provided which works to measure a resistance of the solid electrolyte element. The applying voltage correcting circuit may correct the target voltage as a function of the resistance measured by the resistance measuring circuit.
The second cell outputs a current as a function of the concentration of the specified oxygen containing gas component. A second cell output correcting circuit may be provided which works to correct the current outputted from the second cell based on the output of the monitor cell.
A change rate determining circuit may be provided which works to determine a variable rate at which the target voltage applied to the first cell is to be changed. When the concentration of the gasses changes, so that the concentration of the oxygen molecules changes. The electric current produced by the first cell is, thus changed, thereby causing the target voltage to be applied to the first cell to be changed. In this case, the modification of the rate of application of the target voltage enables the oxygen molecules to be pump into or out of the gas chamber at an increased velocity.
The change rate determining circuit may increase the variable rate as a difference between an actual voltage applied to the first cell and the target voltage to be applied to the first cell increases.
The change rate determining circuit may determine the variable rate by setting a cycle in which the target voltage is changed. The change rate determining circuit may increase the cycle as a difference between an actual voltage applied to the first cell and the target voltage to be applied to the first cell is decreased. This results in an advance in convergence of the voltage applied to the first cell on the target one. The adjustment of the variable rate may eliminate the effect of a peak current produced when the voltage applied to the first cell is changed. Specifically, when the voltage applied to the first cell is changed, a peak current (i.e., tailing) is produced as an output of the first cell, but the adjustment of the variable rate enables the target voltage to be applied to the first cell to be changed after the peak current disappears. FIG. 23 shows an equivalent circuit of the gas concentration sensor. Rg indicates the resistance of particles of a solid electrolyte (zirconia) to oxygen irons. Rh and Ch indicate a grain boundary resistance and a grain boundary capacitance on a boundary face of the solid electrolyte, respectively. Rf and Cf indicate an electrode boundary face resistance and an electrode boundary face capacitance. When the voltage to be applied to the gas concentration sensor is, as shown in FIG. 24, changed, it will cause a peak current to be produced immediately due to charges stored by the capacitances Ch and Cf. The above adjustment of the change rate enables the voltage applied to the first cell to be controlled without the influence of the capacitances of the gas concentration sensor.
A current measuring range is defined in which the electric current produced by the first cell is to be measured. The voltage-to-current relation is defined by a target applying voltage line representing the target applying voltage to be applied to the first cell in terms of the electric current produced by the first cell. The target applying voltage line includes a segment which changes with a change in electric current produced by the first cell at a first inclination substantially depending upon a resistance of the first cell within the current measuring range. Within an outside range defined outside the current measuring range, the target applying voltage line includes a segment which changes at a second inclination reverse in sign to the first inclination. This avoids undesirable generation of heat arising from an excess increase in output of the first cell.
A higher voltage may be applied to the first cell for a period of time following energization of the gas concentration sensor. When the gas concentration sensor is started, the gas chamber is filled with the air, so that an excess quantity of oxygen exist in the gas chamber. The application of the higher voltage causes the excess quantity of oxygen to be discharged out of the gas chamber quickly.
When the output of the monitor cell falls out of a specified range immediately after the gas concentration is energized, the applying voltage controlling circuit applies the higher voltage to the first cell.
According to the second aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a gas chamber, a first cell responsive to application of a voltage to pump thereinto oxygen molecules contained in gasses admitted into the gas chamber and discharge the pumped oxygen molecules to produce an electric current indicative of a concentration of the oxygen molecules, a second cell working to determine a concentration of a specified oxygen containing gas component contained in the gasses having passed through the first cell, and a monitor cell working to monitor a concentration of residual oxygen molecules within the gas chamber and provide an output indicative thereof; (b) an applying voltage correcting circuit working to determine a target voltage to be applied to the first cell by looking up a predetermined voltage-to-current relation as a function of the electric current produced by the first cell, the applying voltage correcting circuit correcting one of the target voltage and the predetermined voltage-to-current relation as a function of the output of the monitor cell; and (c) an applying voltage controlling circuit working to control voltage applied to the first cell into agreement with the target voltage determined by the applying voltage correcting circuit. This enables a residual quantity of the oxygen within the gas chamber to be kept constant regardless of a change in activity of the first cell, thus resulting in improved accuracy of determining the concentration of the specified oxygen containing gas.
In the preferred mode of the invention, the voltage-to-current relation is defined in a map by a target applying voltage line which represents the target applying voltage to be applied to the first cell in terms of the electric current produced by the first cell. The applying voltage correcting circuit corrects the voltage-to-current relation by changing an inclination of the target applying voltage line defined in the map as a function of the output of the monitor cell. For instance, as the output of the monitor cell increases, the inclination is preferably decreased, thereby increasing the target voltage to be applied to the first cell, so that a residual quantity of oxygen within the gas chamber is decreased.
The applying voltage correcting circuit may alternatively correct the voltage-to-current relation by changing an offset of the target applying voltage line in terms of the electric current produced by the first cell as a function of the output of the monitor cell. For instance, as the output of the monitor cell increases, the offset is preferably increased, thereby increasing the target voltage to be applied to the first cell, so that a residual quantity of oxygen within the gas chamber is decreased.
The applying voltage correcting circuit may correct the target voltage to be applied to the first cell so as to bring the output of the monitor cell into agreement with a target value required for keeping the concentration of oxygen molecules at a given level within the gas chamber.
The correction of the target voltage to be applied to the first cell is performed in a cycle longer than that in which the voltage applied to the first cell is controlled by the applying voltage controlling circuit in view of a change in response arising from deterioration or an inherent error of the sensor.
According to the third aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a gas chamber, a first cell responsive to application of a voltage to pump thereinto oxygen molecules contained in gasses admitted into the gas chamber and discharge the pumped oxygen molecules to produce an electric current indicative of a concentration of the oxygen molecules, a second cell working to produce an electric current for determining a concentration of a specified oxygen containing gas component contained in the gasses having passed through the first cell, and a monitor cell working to monitor a concentration of residual oxygen molecules within the gas chamber and provide an output indicative thereof; (b) an applying voltage determining circuit looking up a predetermined voltage-to-current relation to determine a target voltage to be applied to the first cell as a function of the electric current produced by the first cell so as to preclude the first cell from decomposing the specified oxygen containing gas component; and (c) a second cell output correcting circuit working to correct the electric current outputted by the second cell as a function of the output of the monitor cell. This avoid an error in determining the concentration of the specified oxygen containing gas arising from a change in residual quantity of oxygen within the gas chamber.
In the preferred mode of the invention, the second cell output correcting circuit subtracts a current value equivalent to the output of the monitor cell representing the concentration of residual oxygen molecules from the electric current produced by the second cell.
The second cell output correcting circuit corrects the electric current produced by the second cell and the output of the monitor cell as a function of a difference in catalysis between the second cell and the monitor cell, after which the second cell output correcting circuit subtracts the current value equivalent to the output of the monitor cell representing the concentration of residual oxygen molecules from the electric current produced by the second cell.
The second cell and the monitor cell are disposed adjacent to each other and exposed to a second chamber formed downstream of the first cell.
According to the fourth aspect of the invention, there is provided a gas concentration measuring apparatus comprising: (a) a gas concentration sensor including a gas chamber, a first cell responsive to application of a voltage to pump thereinto oxygen molecules contained in gasses admitted into the gas chamber and discharge the pumped oxygen molecules to produce an electric current indicative of a concentration of the oxygen molecules, a second cell working to output an electric current for determining a concentration of a specified oxygen containing gas component contained in the gasses having passed through the first cell, and a monitor cell working to monitor a concentration of residual oxygen molecules within the gas chamber and provide an output indicative thereof; (b) an applying voltage determining circuit working to determine a target voltage to be applied to the first cell by looking up a predetermined voltage-to-current relation as a function of the electric current produced by the first cell; (c) a first correcting circuit working to correct one of the target voltage and the predetermined voltage-to-current relation as a function of the output of the monitor cell so as to preclude the first cell from decomposing the specified oxygen containing gas component; (d) an applying voltage controlling circuit working to control voltage applied to the first cell into agreement with the target voltage provided by the applying voltage correcting circuit; and (e) a second correcting circuit working to correct the electric current produced by the second cell as a function of the output of the monitor cell to determine the concentration of the specified oxygen containing gas component. This avoid an error in determining the concentration of the specified oxygen containing gas arising from a change in residual quantity of oxygen within the gas chamber.
In the preferred mode of the invention, the correction of the target voltage to be applied to the first cell is performed by the first correcting circuit in a cycle longer than that in which the voltage applied to the first cell is controlled by the applying voltage controlling circuit.
When the output of the monitor cell is brought into agreement with a target value or falls within a range around the target value under control of the target voltage applied to the first cell by the applying voltage controlling circuit, the second correcting circuit corrects the electric current produced by the second cell based on the output of the monitor cell.
According to the fifth aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a gas chamber, a first cell responsive to application of a voltage to pump thereinto oxygen molecules contained in gasses admitted into the gas chamber and discharge the pumped oxygen molecules to produce an electric current indicative of a concentration of the oxygen molecules, a second cell working to determine a concentration of a specified oxygen containing gas component contained in the gasses having passed through the first cell, and a monitor cell working to monitor a concentration of residual oxygen molecules within the gas chamber and provide an output indicative thereof, the first cell being formed in a solid electrolyte element; (b) a resistance determining circuit working to determine a resistance of the solid electrolyte element; (c) an applying voltage determining circuit working to determine a target voltage to be applied to the first cell by looking up a predetermined voltage-to-current relation as a function of the electric current produced by the first cell; (d) an applying voltage correcting circuit correcting one of the target voltage and the predetermined voltage-to-current relation as a function of the resistance determined by the resistance determining circuit; and (e) an applying voltage controlling circuit working to control voltage applied to the first cell into agreement with the target voltage provided by the applying voltage correcting circuit. This avoid an error in determining the concentration of the specified oxygen containing gas arising from a change in residual quantity of oxygen within the gas chamber.
In the preferred mode of the invention, the voltage-to-current relation is defined in a map by a target applying voltage line which represents the target applying voltage to be applied to the first cell in terms of the electric current produced by the first cell. The applying voltage correcting circuit corrects the voltage-to-current relation by changing an inclination of the target applying voltage line defined in the map as a function of the resistance of the first cell.
The voltage-to-current relation is defined in a map by a target applying voltage line which represents the target applying voltage to be applied to the first cell in terms of the electric current produced by the first cell. The applying voltage correcting circuit corrects the voltage-to-current relation by changing an offset of the target applying voltage line in terms of the electric current produced by the first cell as a function of the resistance of the first cell.